U.S. oil refineries that are unable to sell a dirty fuel waste product at home are exporting vast quantities of it to India instead.

Petroleum coke, the bottom-of-the-barrel leftover from refining Canadian tar sands crude and other heavy oils, is cheaper and burns hotter than coal. But it also contains more planet-warming carbon and far more heart- and lung-damaging sulfur -- a key reason few American companies use it.

Refineries instead are sending it around the world, especially to energy-hungry India, which last year got almost a fourth of all the fuel-grade "petcoke" the U.S. shipped out, an Associated Press investigation found.

In 2016, the U.S. sent more than 8 million metric tonnes of petcoke to India. That's about 20 times more than in 2010, and enough to fill the Empire State Building eight times.

The petcoke being burned in countless factories and plants is contributing to dangerously filthy air in India, which already has many of the world's most polluted cities.

Laboratory tests on imported petcoke used near New Delhi found it contained 17 times more sulfur than the limit set for coal, and a staggering 1,380 times more than for diesel, according to India's court-appointed Environmental Pollution Control Authority. India's own petcoke, produced domestically, adds to the pollution.

Industry officials say petcoke has been an important and valuable fuel for decades, and its use recycles a waste product. Health and environmental advocates, though, say the U.S. is simply exporting an environmental problem. The U.S. is the world's largest producer and exporter of petcoke, federal and international data show.

"We should not become the dust bin of the rest of the world," said Sunita Narain, a member of the pollution authority who also heads the Delhi-based Center for Science and the Environment. "We certainly can't afford it; we're choking to death already."

Petcoke traditionally was used in the U.S. to make aluminum and steel after its impurities were removed. But when those mills closed or moved to other countries, the need for petcoke waned, although some power plants still use it.

The American Fuel and Petrochemical Manufacturers, a petroleum industry trade group, released a statement to the AP saying that cokers "allow the United States to export petroleum coke to more than 30 countries to meet growing market demand."

But experts say it's not market forces that are driving U.S. refiners to make this waste product from heavy oil refining. The refineries just need to get rid of it, and are willing to discount it steeply -- or even take a loss -- which helps drive the demand in developing countries, experts said.

Petcoke, critics say, is making a bad situation worse across India. About 1.1 million Indians die prematurely as a result of outdoor air pollution every year, according to the Health Effects Institute, a nonprofit funded by the U.S. Environmental Protection Agency and industry.

In New Delhi, pollution has sharply increased over the past decade with more cars, a construction boom, seasonal crop burning and small factories on the outskirts that burn dirty fossil fuels with little oversight.

In October and November, for the second year in a row, city air pollution levels were so high they couldn't be measured by the city's monitoring equipment. People wore masks to venture out into gray air, and newspaper headlines warned of an "Airpocalypse."

The country has seen a dramatic increase in sulfur dioxide and nitrogen dioxide emissions in recent years, concentrated in areas where power plants and steel factories are clustered. Those pollutants are converted into microscopic particles that lodge deep in the lungs and enter the bloodstream, causing breathing and heart problems.

It's impossible to gauge precisely how much is from petcoke versus coal, fuel oil, vehicles and other sources. But experts say it certainly is contributing.

Indian purchases of U.S. fuel-grade petcoke skyrocketed two years ago after China threatened to ban the import of high-sulfur fuels. Although Indian factories and plants buy some petcoke from Saudi Arabia and other countries, 65 per cent of imports in 2016 were from the U.S., according to trade data provider Export Genius.

India's cement companies were first to bring in petcoke, and still import the most, though cement experts say some sulfur is absorbed during manufacturing.

As word spread of the cheap, high-heat fuel, other industries began using it in their furnaces -- producing everything from paper and textiles to brakes, batteries and glass, according to import records compiled by Export Genius.

The government was caught off guard by the shift, and there are scant records of how much petcoke is being burned. Petcoke's use was further encouraged by low import tariffs and a lack of regulations on its most potent pollutants.

Industries also like that petcoke, which is around 90 per cent carbon, burns hot. So they can use less of it to produce the same heat as coal -- though coal still overshadows petcoke in factory furnaces.

Within a decade, India's petcoke appetite grew so voracious that it began producing and selling its own, and Indian refineries today are making about as much as the country is importing. One of the biggest refiners -- Mumbai- based Reliance Industries Ltd., owned by India's wealthiest businessman, Mukesh Ambani -- has ramped up petcoke production.

Judges of India's National Green Tribunal demanded in May that the government investigate the environmental and health impacts of petcoke.

The government's environment ministry has dismissed the idea that petcoke threatens public health in the nation's capital.

But the country's Supreme Court, which has consistently demanded or enacted tougher pollution control measures, recently banned petcoke use by some industries as of November 1 in the three states surrounding pollution-choked New Delhi. It also demanded tighter pollution standards that -- if enforced -- could further limit its use nationwide.

"This is a completely disgusting state of affairs," the judges said in their (October 24) ruling, "and this is hardly the way in which the Ministry ought to function if it is expected to perform its duties sincerely, honestly and with dedication."

The court last month also urged all states across India to pass similar bans.

"The petcoke markets grew so fast across the country that a ban around New Delhi isn't going to put a huge dent in the overall demand for petcoke," said Jeffrey McDonald, an analyst at S&P Global Platts.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)